This invention relates to voltammetric apparatus, more particularly improved apparatus for use in differential pulse anodic stripping voltammetry (hereinafter abbreviated as DPASV).
DPASV is recognized in the art as an effective method for analyzing a minute quantity of components, especially metals contained in solutions.
The method of DPASV consists essentially of the following two steps. In the first step a plating voltage about several hundreds millivolts negative with reference to the oxidation reduction potential of the component to be analyzed is impressed upon a working electrode immersed in a solution to be measured for plating the working electrode. In the second step a voltage comprising direct current voltage (stripping voltage) which increases gradually with time in the positive direction from the plating voltage and superimposed upon a pulse voltage is applied by sweeping upon the working electrode for stripping the component that has been plated on the working electrode during the first step. For this reason, the second step is termed the stripping step. FIG. 1 of the accompanying drawing is a graph showing the manner of applying the voltage in the first and second steps. Although in FIG. 1 the pulse voltage is shown as being superimposed upon the stripping voltage (sweep potential) on the positive side thereof, it is also possible to superimpose the pulse voltage on the negative side of the stripping voltage.
In sweeping during the second step, among the current flowing through the working electrode, the current that flows while the pulse is not applied, preferably the current that flows during definite interval (sampling time S.sub.1) immediately prior to the application of the pulse voltage, and the current that flows while the pulse is applied, preferably a definite interval of the latter half of the pulse (sampling time S.sub.2) are measured to obtain sampling currents IS.sub.1 and IS.sub.2 respectively and the difference between these sampling currents is determined. FIG. 2 diagrammatically shows one example of the relationship between the pulse and the sampling times S.sub.1 and S.sub.2.
To calculate the difference between sampling currents IS.sub.1 and IS.sub.2, these currents are usually converted into corresponding sampling voltages which are held by respective voltage holders and then the difference between these sampling voltages is determined. The maximum value or the integrated value of the voltage difference is used to calculate the concentration of the component to be analyzed.
In this case curves representing sample currents IS.sub.1 and IS.sub.2 are illustrated in FIG. 3. As has been described above where the pulse voltage is superimposed on the negative side of the stripping voltage curve S.sub.2 will be positioned beneath curve S.sub.1 with its polarity reversed. The difference between sample currents IS.sub.2 and IS.sub.1, that is the output signal is shown by FIG. 4.
The outline and present state of DPASV can be found in the following papers.
a. J. B. Flato, Analytical Chemistry, Vol. 44, September, 1972, pages 75A-87A.
b. H. Seigerman et al., Americal Laboratory, Vol. 4, No. 6, pages 59-68 (1972)
c. T. R. Copeland et al., Analytical Chemistry, Vol. 46, No. 14, Dec. 1974, pages 1257A-1264A.
One example of the apparatus for use in DPASV is described in U.S. Pat. No. 3,420,764.
Although DPASV is an analytical method having an extremely high sensitivity, it is desired to improve further the sensitivity.
Thus, the prior art apparatus for use in DPASV has such problems as poor separation of the peaks obtained on a recording paper, asymmetrical shape of the peaks and low measuring accuracies of various components. Moreover, both ends of the base line of the peaks recorded on a recording paper tend to shift upwardly thus forming a dish like base line. Consequently, the measuring accuracy of the peaks of certain components such as copper and zinc appearing on both ends decreases. The output signal obtainable in such a case is illustrated in FIG. 5.
As a result of our investigation we have found that these difficulties can be improved greatly by improving the filter circuit utilized in the voltage holding circuit.